The last decade has seen a bonanza of exoplanet discoveries. Nearly
2,000 exoplanets -- planets outside our solar system -- have been
confirmed so far, and more than 5,000 candidate exoplanets have been
identified. Many of these exotic worlds belong to a class known as "hot
Jupiters." These are gas giants like Jupiter but much hotter, with
orbits that take them feverishly close to their stars.

At first, hot Jupiters were considered oddballs, since we don't have
anything like them in our own solar system. But as more were found, in
addition to many other smaller planets that orbit very closely to their
stars, our solar system started to seem like the real misfit.

"We thought our solar system was normal, but that's not so much the
case," said astronomer Greg Laughlin of the University of California,
Santa Cruz, co-author of a new study from NASA's Spitzer Space Telescope
that investigates hot Jupiter formation.

As common as hot Jupiters are now known to be, they are still
shrouded in mystery. How did these massive orbs form, and how did they
wind up so shockingly close to their stars?

The Spitzer telescope found new clues by observing a hot Jupiter
known as HD 80606b, situated 190 light-years from Earth. This planet is
unusual in that it has a wildly eccentric orbit almost like that of a
comet, swinging very close to its star and then back out to much greater
distances over and over again every 111 days. One side of the planet is
thought to become dramatically hotter than the other during its
harrowing close approaches. In fact, when the planet is closest to its
host star, the side facing the star quickly heats up to more than 2,000
degrees Fahrenheit (1,100 degrees Celsius).

"As the planet gets closer to the star, it feels a burst of
starlight, or radiation. The atmosphere becomes a cauldron of chemical
reactions, and the winds ramp up far beyond hurricane force," said
Laughlin, a co-author on the Spitzer study, which is accepted for
publication in The Astrophysical Journal.

HD 80606b is thought to be in the process of migrating from a more
distant orbit to a much tighter one typical of hot Jupiters. One of the
leading theories of hot-Jupiter formation holds that gas giants in
distant orbits become hot Jupiters when the gravitational influences
from nearby stars or planets drive them into closer orbits. The planets
start out in eccentric orbits, then, over a period of hundreds of
millions of years, are thought to gradually settle down into tight,
circular orbits.

"This planet is thought to be caught in the act of migrating inward,"
said Julien de Wit of the Massachusetts Institute of Technology,
Cambridge, lead author of the new study. "By studying it, we are able to
test theories of hot Jupiter formation."

Spitzer previously studied HD 80606b in 2009. The latest observations
are more detailed, thanks to a longer observing time -- 85 hours -- and
improvements in Spitzer's sensitivity to exoplanets.

"The Spitzer data are pristine," said de Wit. "And we were able to
observe the planet for much longer this time, giving us more insight
into its coldest temperature and how fast it heats up, cools down and
rotates."

A key question addressed in the new study is: How long is HD 80606b
taking to migrate from an eccentric to a circular orbit? One way to
assess this is to look at how "squishy" the planet is. When HD 80606b
whips closely by its star, the gravity of the star squeezes it. If the
planet is squishier, or more pliable, it can better dissipate this
gravitational energy as heat. And the more heat that is dissipated, the
faster the planet will transition to a circular orbit, a process known
as circularization.

"If you take a Nerf ball and squeeze it a bunch of times really fast,
you'll see that it heats up," said Laughlin. "That's because the Nerf
ball is good at transferring that mechanical energy into heat. It's
squishy as a result."

The Spitzer results show that HD 80606b does not dissipate much heat
when it is squeezed by gravity during its close encounters - and thus is
not squishy, but rather stiffer as a whole. This suggests the planet is
not circularizing its orbit as fast as expected, and may take another
10 billion years or more to complete.

"We are starting to learn how long it may take for hot Jupiter
migration to occur," said de Wit. "Our theories said it shouldn't take
that long because we don't see migrating hot Jupiters very often."

"The long time scales we are observing here suggest that a leading
migration mechanism may not be as efficient for hot Jupiter formation as
once believed," said Laughlin.

The Spitzer study suggests that competing theories for hot Jupiter
formation -- in which gas giants form "in situ," or close to their
stars, or smoothly spiral inward with the help of planet-forming disks
-- may be preferred.

The new study is also the first to measure the rotation rate of an
exoplanet orbiting a sun-like star. Spitzer observed changes in the
planet's brightness as the planet spun on its axis, finding a rotation
period of 90 hours.

"Fifty years ago, we were measuring the rotation rates of planets in
our own solar system for the first time. Now we are doing the same thing
for planets orbiting other stars. That's pretty amazing," said
Laughlin.

A rotation rate of 90 hours is much slower than what is predicted for
HD 80606b, puzzling astronomers, and adding to the enduring mystique of
hot Jupiters.

Additional study authors are: Nikole Lewis of the Space Telescope
Science Institute in Baltimore; Jonathan Langton of Principia College,
Elsah, Illinois; Drake Deming of University of Maryland, College Park;
Konstantin Batygin of the California Institute of Technology, Pasadena;
and Jonathan Fortney of the University of California, Santa Cruz.